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Reduced mass

Reduced mass, often written μ, is a concept that allows one to solve the two body problem of mechanics as if it were a one body problem. Given two bodies, one with mass M and the other mass m, the equivalent one body problem is that of a single body of mass
orbiting the center of mass of the two bodies. If one body is much more massive than the other, it is useful to think of that body as fixed in space and the other body as having mass μ. As the name suggests, the reduced mass will always be less than the actual mass of the body.